1. Technical Field
Technical Field: The present invention relates to an electric supply apparatus for a high pressure discharge lamp which can be suitably used as a light source of a liquid crystal projector apparatus which has a light modulation function.
2. Related Art
In a projection type projector apparatus, there is a demand for an image with uniform and sufficient color reproduction nature, which is projected on a rectangular screen. For this reason, a short arc type high pressure discharge lamp, in which the mercury vapor pressure thereof reaches, for example, 150 atmospheres or more when it is lit, is adopted as a light source and is, for example, lighted in an alternating current lighting method when steady lighting is used. In recent years, a projector apparatus, which has a light modulation function capable of adjusting the brightness of a screen according to the brightness of the environment or a kind of image to be projected, has been developed, in which, for example, the so-called “brightness adjustment mode” using a dimming function so as to raise contrast by decreasing electric power according to such a screen, or a low power lighting modes such as the so-called “super energy saving mode” etc. for decreasing electric power is adopted. In such a projector apparatus, it is required that electric power in a low power lighting mode be reduced to 25-70% of the rated power.
However, in the low power lighting mode, the position of the arc in the light source becomes unstable and a flicker tends to occur, because the tip of an electrode decreases in temperature due to the decrease of the electric power applied to the electrode tip. Japanese Patent Application Publication No. 2010-238526 discloses that, in order to solve such a problem, in a low power lighting mode in which electric power of, for example, 70% or less of the rated power is supplied to a high pressure discharge lamp to light the lamp, base lighting and boost lighting are performed by turns, wherein in the base lighting, base current having a predetermined frequency is supplied to the lamp, and in the boost lighting, boost current whose value is larger than a value of the base current is supplied thereto.
On the other hand, there is a known problem in which, when electric power to be supplied is increased so as to light such a discharge lamp in alternating current lighting, since the luminance of the lamp increases as the electric power is increased, areas with different brightness in a projection image are generated, so that a horizontal stripe noise is perceived on the projection image which is projected on a screen. In order to solve such a problem, Japanese Patent Application Publication No. 2009-198886 discloses, as shown in FIG. 7, that in a liquid crystal projector, in which a liquid crystal panel is irradiated with light emitted from the light source driven by alternating current (thereby generating a projection image light), in a period (pulse period t1) in which boost lighting is carried out, the luminance of a video signal is decreased by a predetermined amount ΔL based on a vertical synchronizing signal for driving a panel, which determines a vertical blanking period of the liquid crystal panel, whereby generation of the horizontal stripe noise of the projection image which is projected on a screen is supposed to be prevented while generation of a flicker is suppressed.
However, just as in the discharge lamp lighting apparatus disclosed in Japanese Patent Application Publication No. 2010-238526, it turns out that in the discharge lamp lighting apparatus disclosed in Japanese Patent Application Publication No. 2009-198886 areas of different brightness are generated by carrying out boost lighting in the low power lighting mode, in which electric power smaller than the rated power is supplied to a high pressure discharge lamp in order to light the discharge lamp. The present inventors made a prototype of a liquid crystal projector apparatus equipped with a lamp lighting apparatus using the technology disclosed in Japanese Patent Application Publication No. 2009-198886, wherein the high pressure discharge lamp is lighted in the low power lighting mode, in which, electric power lower than the rated power is supplied to the high pressure discharge lamp based on constant electric power control and wherein the transmittance of a liquid crystal panel is controlled in the low power lighting mode. Then, the present inventors confirmed that the above-mentioned problem were not be solved by the technology disclosed in Japanese Patent Application Publication No. 2009-198886. The reasons therefor are set forth below.
Since the temperature of vapor in the electrical discharge space and the temperature of the electrodes are low when the high pressure discharge lamp is lighted in the low power lighting mode, mercury in the electrical discharge space is unsaturated, that is, part of the mercury aggregates, so that it is necessary to take time to evaporate the mercury. Therefore, a change of lamp lighting power cannot be actually predicted in response to a steep change of a current supply command signal, when the high pressure discharge lamp is lighted in the boost lighting. For this reason, even when the current supply command signal is transmitted so that the boost current, which is obtained by superimposing current having a predetermined constant magnitude on the base current, is supplied to the discharge lamp in the boost lighting period of a predetermined fixed cycle, the lamp current that actually flows through the high pressure discharge lamp changes in every boost lighting period, so that desired boost lighting cannot be performed. And since the luminance of light emitted from the discharge lamp depends on lamp power, and this lamp power is determined by the lamp current that actually flows through the discharge lamp and the lamp lighting voltage, the lamp lighting power WL also changes in every boost lighting period, resulting in the luminance of the light emitted from the high pressure discharge lamp varying in every boost lighting period. Accordingly, the luminance is not constant and it is impossible to know (predict) the luminance thereof in advance.
Therefore, in the technology disclosed in Japanese Patent Application Publication No. 2009-198886, in which the luminance of the light emitted from the high pressure discharge lamp is decreased by a predetermined constant amount ΔL at predetermined times in a fixed boost lighting period in the boost lighting mode, since a state of the boost lighting to be controlled varies in every boost lighting period in practice, light (or the luminance thereof) cannot be sufficiently reduced in a certain boost lighting period, or it is reduced too much in another boost lighting period, so that areas with different brightness in a projection image differs are generated.